Wrinkle finish powder coating compositions are dry, free flowing powders applied in fusion coating processes. A fusion coating process is a process in which a coating powder is distributed over a substrate and, when heated, fuses to form a continuous film. The substrate may be heated or unheated when the powder is applied thereto. Heat supplied from the substrate or from an external source, such as an oven, causes the powder to fuse into the continuous film. Known fusion coating processes for application of powder coating compositions to a substrate include electrostatic spraying, fluidized bed coating and hot flocking.
Wrinkle finish coatings fuse to form a film which has a uniformly random series of irregularities which resemble wrinkles. Although having a generally similar “look” or overall appearance, a careful study of a wrinkle finish film reveals what are essentially unique non-repetitive random patterns.
Wrinkle finish powder coating compositions may be made of many different resin materials including epoxy, epoxy-polyester, urethane-polyester, TGIC-free polyester, and acrylic coating materials. TGIC-free refers to resins which are free of triglycidylisocyanurate. Other constituents typically include curatives, flow aids, degassing agents, catalysts, pigments, fillers and charge inhibitors.
Wrinkle finishes are ideal for applications in which hiding of scratches, fingerprints, stains and the like on the coated substrate is desired. Exemplary applications include coating of metal industrial and office equipment, such as toolboxes, file cabinets and computer cabinets. Other examples include lighting fixtures, metal toys and sporting goods.
A potential problem facing applicators of wrinkle finish powder coating materials involves formation in the film of a type of surface anomaly or imperfection known generally in the industry as a “starburst.” A starburst or starburst-type anomaly is defined as a surface imperfection which has a somewhat starburst-like appearance. The anomaly typically resembles a series of ripples projecting radially outward from a starburst center. Starburst anomalies may also appear as shiny or glossy spots in the finished film. FIG. 1 is a “Q-panel” with superimposed circles around representative starburst anomalies.
Starburst anomalies may appear randomly along the film formed by the coating. The starburst anomalies disrupt the unique, non-repetitive random patterns of the wrinkle finish and, therefore, affect the random wrinkled appearance of the film. For some applications, starburst-type anomalies are of no consequence and are perfectly acceptable. However, for certain other applications, starburst-type anomalies in the finished film must be controlled. The presence of such anomalies in the finished film may result in rejection of the coated article by the customer. Any such rejection may result in discarding of many thousands of wrinkle finish coated articles or in costly reworking of such articles.
Typical efforts to minimize starbursts have been directed toward improving powder coating material flow characteristics. Such efforts have included increasing extruder temperatures and mix times and increasing flow additives. However, these efforts have not been entirely satisfactory because they may result in undesired side effects such as a condition known as “edge pulling.” Edge pulling is a condition in which the coating pulls away from the corners of the coated substrate resulting in incomplete formation of the finished film.